not of sense-substances only, but of living substance generally,.
Each cell has its specific energy in Johannes Müller's sense, and
in its extended form there is no more general law for all the
operations of stimuli than this law of specific energy. To take
examples, whether a muscle be stimulated by a chemical,
mechanical, thermal or electrical stimulus the result is in each
case the same—namely, a twitching of the muscle. Let a salivary
gland be stimulated chemically, mechanically, electrically or
in any other way, there always follows the same specific action—a
secretion of saliva, no matter what be the kind of stimulus
acting upon it, the liver-cell always reacts by producing bile,
and so on. On the other hand, one and the same stimulus—the
electric current, for example—gives in each form of living
substance a specific result: twitching in the muscle, secretion
of saliva in the salivary gland, production of bile in the liver-cell,
&c. That is, of course, with the proviso that the effect of the
stimulus be exciting and not depressing. The following general
formulation, however, of the law of specific energy brings the
depressing stimuli also within its scope: “Different stimuli
produce in each form of living substance an increase or a diminution
of its specific activity.” As already observed, it will
probably be found that those weak chronic forms of stimulation
which produce qualitative changes may also be comprised under
this general law.
The knowledge thus far acquired from analysis of vital phenomena and their changes under the influence of stimuli Mechanism of Life. affords but a very indefinite temporary basis for the theory of the actual vital process itself, of which vital phenomena are the outward manifestation. The conceptions to which physiological research has hitherto attained in this matter are of a more or less doubtful nature. The facts contained in them still require to be linked together by hypotheses if we are to obtain even a vague outline of what lies hidden behind the great riddle of life. Such hypotheses, serving as they do to link facts consistently together, are absolutely essential, however, to the further progress of research, and without their aid any systematic investigation would be impracticable. But at the same time it must never be forgotten that these hypotheses are merely provisional, and that whenever they are found to be no longer in harmony with the widening range of new experiences and ideas they must either be proved to be facts or be subjected to modification. This is the point of view from which we must deal with modern ideas concerning the nature of the actual vital process—the mechanism of life
The fundamental fact of life is the metabolism of living substance which is continually and spontaneously undergoing Metabolism. decomposition, and building itself up anew with the help of the food-substances it takes in. These processes of decomposition and of reconstruction may be briefly designated as dissimulation (catabolism) and assimilation (anabolism) respectively. Now the question arises: How are we to understand this process of dissimulation and assimilation from a mechanical standpoint? It is quite evident that we have to do with some chemical occurrence; but how are the chemical transformations brought about? There are obviously two possibilities. It is conceivable that the decomposition of food-stuffs and the formation of excretion-products in the cell-body are caused by the repeated casual encounter of a great series of chemical combinations and by their repeatedly reacting upon one another in the same manner, bringing about transformations and forming waste products which are excreted, while at the same time certain chemical affinities are always taking in from without new chemical combinations (food-stuffs) and uniting them. This theory was in fact occasionally advanced in former times, particularly in its chemical aspect, and the belief was especially entertained that the enzymes in living substance might play an important part in these transformations. This assumption, however, leads to no clear and lucid image of what takes place, and, moreover, draws too largely upon auxiliary hypotheses. It has therefore met with but little acceptance. The other possible explanation of metabolism is that its whole process is confined to one single class of chemical combinations whose tendency it is to be constantly undergoing spontaneous decomposition and regeneration. This latter theory was founded by Ludimar Hermann (b 1838), Eduard Friedrich Pflüger (b. 1829) and others, and has met with universal recognition because of its naturalness, simplicity and clearness.
Starting with this hypothesis, the path of further research lies clear and well defined before us. In the first place, we are Proteids. obviously met by the question: What conception are we to form of these combinations on which hinges the whole vital process? Among the organic matters which compose living substance, proteids perform the most important part. Proteids and proteid-compounds form the only organic matter which is never absent from any cell. They form also the greater part of all the organic compounds of the cell, unless reserve-stuffs are accumulated to a considerable extent, and they are by far the most complicated of the compounds of living substance. While animal life is impossible without proteid food, there are, on the other hand, animals which can continue to subsist on proteid alone. This series of facts proves very conclusively that proteids and their compounds play by far the most important part of all organic matter in the processes of life. The idea thus naturally presents itself that the required hypothetical compound forming the central point of metabolism will be found to bear a very close relation to proteids. But another point must be here considered. The proteids and their compounds known to us are, comparatively speaking, stable compounds, which never undergo spontaneous decomposition so long as they are protected from outward injury, whereas the hypothetical combination which lies at the centre of organic metabolism is extraordinarily liable and continually undergoing spontaneous decomposition. Therefore we have to think not of ordinary proteids in this case, but of still more complicated combinations, the atoms in the molecule of which have a strong tendency to group themselves in new arrangements. Owing to their fundamental importance, these combinations have been termed “biogens.” When we come to inquire how such labile biogen molecules are built up out of the proteids of food, we find our knowledge very much restricted. Doubtless the intramolecular addition of inspired oxygen has much to do with it; for living substance when deprived of oxygen loses its irritability—that is to say, its tendency to decomposition. The fact that the decomposition of living substance is always associated with the formation of carbonic acid—a circumstance obviously necessitating the aid of oxygen—also points to the absolute indispensableness of oxygen in the matter. Pflüger has further suggested that the molecule of living substance owes its lability and its tendency to form carbonic acid when joined by oxygen atoms principally to cyanogen groups which are contained in it. According to this view, the following is supposed to be the process of the formation of biogen molecules: It is assumed that the biogen molecules already present in living substance take out of the proteids of food certain groups of atoms, and dispose them so as to produce cyanogen-like compounds. The addition of oxygen atoms then brings the biogen molecule to the maximum of its power of decomposition, so that—partly spontaneously, but more especially when impelled by a stimulus—it breaks down somewhat explosively, causing the formation of carbonic acid. In this proceeding, according to the hypothesis which is the most widely accepted and the most fruitful in results, would lie the very germ of the vital process.
If we accept these views as far as their general principle is concerned, assimilation is the re-formation of biogen molecules Mechanism of Cell-life. by those already existing, aided by food-stuffs; dissimulation, the decomposition of biogen molecules. To this primary process, however, is attached a whole series of secondary chemical processes, which serve partly to work upon the food so as to fit it for the building up of biogen molecules, and partly to form out of the direct decomposition-products of the biogen molecules the characteristic secretion-products of living substance (excretions and secretions). The various workings of matter in the cell are rendered very much more
